Peavey M7000 won't energize output relays

[Lynx]

Instead of hijacking this thread, Peavey M7000 Power amp , I'll just start a new thread instead.

As a very simple test, would it be possible to separate the power amp from the preamp and determine if the fault lies in the one or the other?
I'm thinking if I completely separate the power amp and feed it with 2 separate power supplies, which would give me between +30 to 0 to -30 Volts and then tie both the positive and negative rail controlling transistors to ground, if that would lead to getting zero Volts on both channel outputs, would I then be able to say that the power amp is OK?

So this test showed +250mV DC on the former good channel and +500mV on the other one, I'm guessing those aren't all too high voltages for the DC monitoring circuits to stop the relay(s) from energizing, or are they....?

Next I'll try to connect the native +/- 70V and ground to the power amp and do the same test and see what happens to a 10 Ohm resistor when I connect it to the output.

Would it be possible to connect a sine wave generator directly to the controlling transistors on the power amp and check the output with an oscilloscope to see if it follows the sine wave, or possibly even connect a speaker directly to the output and see, I mean hear, what gives?

[Enzo]

No. No resistors or other loads until we KNOW the amp is right.

Two channels, and neither relays on? Or just one.

I have never in my life had to power a power amp circuit from bench supplies. Power supplies are straightforward to repair. get them right and go from there.

AMp output has to be taken before the relay - left of it on the drawing. Note the protective Q19 SAC187 crowbar triac is inboard of the relay, but its detector is post relay. it is there to protect speakers as well as the relay. SO make sure the SAC187 is not shorted.

Looking for DC on the output, measure from the inboard side of the relay. A convenient spot is any of those 10w 0.33 ohm resistors. Or find L1 and R43 and get it there.

The relay drive is a complicated circuit, drawing from a number of places in the amp, not merely offset voltage. it is all that stuff right above the power transformer and primary wiring.

What the amp does on 30v rails instead of 70v rails is not really of use. It is the equivalent of turning down the 120v mains to 50v.

[Lynx]

Two channels, and neither relays on? Or just one.

It used to be only one relay off, until I started comparing voltages on the transistors controlling the relay, all of the sudden the "working" relay de-energised and refused to come back on again, my bad

AMp output has to be taken before the relay - left of it on the drawing. Note the protective Q19 SAC187 crowbar triac is inboard of the relay, but its detector is post relay. it is there to protect speakers as well as the relay. SO make sure the SAC187 is not shorted.

Looking for DC on the output, measure from the inboard side of the relay. A convenient spot is any of those 10w 0.33 ohm resistors. Or find L1 and R43 and get it there.

The relay drive is a complicated circuit, drawing from a number of places in the amp, not merely offset voltage. it is all that stuff right above the power transformer and primary wiring.

What the amp does on 30v rails instead of 70v rails is not really of use. It is the equivalent of turning down the 120v mains to 50v.

OK, I'll connect the poweramp to +/- 70V and ground then using the same method as with the bench supplies and check for DC offsets on the 0.33 Ohm resistors or L1/R43, thanks for the tips

[Lynx]

Repeating the same test as above using the full +/- 70VDC power supply with the control connectors disconnected from the preamps and pins 3 and 13 tied to ground gave +540mV output voltage on one channel, measured at L1/R43/.33 Ohm resistors and +170mV on the other, both measured in relation to ground.







With both preamps connected I get the same values as before, +28V on one channel and +69V on the other, indicating that the problem(s) could be found somewhere in the preamps (maybe).

Measuring the voltage between base/emitter on Q12 gives about 640 mV, indicating that the differential amplifier stage (Q12/Q13) could be active, despite 0V input to the preamp.

Misc measurements: U2 (RA3678C) pin5 1150mV, pin6 1756mV, pin7 1305mV. U1 (RA3678C) pin5 1187mV pin6 1289mV pin7 1308mV.

So do I remove U1 and U2 and see if the poweramp output voltages drops or do i test U1/U2 separately on a breadboard....?
Otherwise what measurements should be done next?

[Lynx]

So, by removing both U1 and U2 from both preamps the output voltages not only dropped, leaving -1.33V on one and -0.5V on the other, but both output relays also energized, leaving "only" the DDT LED's lit, but for now I'd be happy if I could get both channels to start playing music again as planned.

Would it be naive of me to just assume that the DC voltage problems would go away if I downright just replaced U1 and U2 on both preamps, or could the root cause still be found elsewhere on the preamps, somewhere before U1/U2?

[Lynx]

Performing a unity gain voltage follower test as per application notes for NE5532 on a breadboard showed that all the OP amps actually works the way they should work, with a swing from about 1.3V up to 14.4V, which from what I can see is perfectly OK, atleast according to the datasheet charts anyway.

http://www.ti.com/lit/ds/symlink/ne5532.pdf

Would the next step be to remove Q12 and Q13 and bench test them separately?
I'm slowly running out of ideas here

[Jazz P Bass]

With everything connected, what is the output voltage of U2B/ pin #7?

It should be zero volts or close to that.

A leaky Q12/ Q13 may throw off the readings, so it may help to remove them for the U2 test.



You may have to replace the diffy pair if the output is zero.

[Lynx]

Well, learned a new lesson today.....

When it says +/-15V in the schematics then that doesn't mean that my voltmeter should read 15V when I measure the supply voltage on the OP amps, it should in fact read 30V

Long story short, on one preamp the transistor Q34 (MPS6533) had fried, replaced that one with a 2N4403 and on the other amp power resistor R93, 1.5kΩ/5W had blown, so by "simply" replacing those the control circuitry got full +15/0/-15V to work with, so now both output relays energize as planned and also none of the DDT LED's lights up anymore, so I guess I got off easy this time.

Took it back to the owner who tested it and now it plays music just fine again, on both channels.

Thank you very much everyone, stoked to have found a forum where one actually can get help when you need it, mucho kudos

[Enzo]

The voltage readings are with respect to ground. If you measure +15 to -15, yes, you get 30.

Glad we could help.

[TomCarlos]

Can I ask a follow up question on this one? I see VR1 (7815) and that it supplies the +15v. Unlike other Peavey amps that have a negative regulator, I don't see that here. So the -15 volts is coming from the -70 supply through R93?

[Lynx]

Can I ask a follow up question on this one? I see VR1 (7815) and that it supplies the +15v. Unlike other Peavey amps that have a negative regulator, I don't see that here. So the -15 volts is coming from the -70 supply through R93?

That's how I also understand it, the -15V comes from R93/Q33/Q34/R95/R96, with the +15V regulating it all, or that's what it looks like to me anyway.
It's a most interesting circuit actually, would be fun to replicate it separately on a breadboard, see what gives.